What my Dad taught me about Torque

(Written for a certain crowd)

Effort or resistance torque = force x distance, right? and effort torque counters resistance torque by definition.
If you drive a car, particularly manual, think this:

Effort Torque:
First gear of a car is physically the biggest (large moment arm) so the same acceleration (effort force) gives more power (effort torque) through first gear than second gear (smaller moment arm). You can take off in second but you have to accelerate (effort force) harder to get anywhere (effort overcoming resistance).

Equally if we’re cruising along at 80km/h and change down a gear (increase size of moment arm) and keep accelerating the same, we will have more power to counter resistance torque and will speed up (think Fast and Furious gear dropping).

So in a car:
Effort Torque = acceleration x gear size
– obviously if we add acceleration or size we have more torque, vica versa


Resistance Torque
Say we add a trailer to the car or take off against gravity up hill, we are adding push power (resistance torque) against the engine (effort torque).

So we need to increase our effort torque to be able to create the same take off:
– use a lower gear (effort moment arm)
– add acceleration (effort force)
– move the impact of the car’s weight closer to the center of the gear (resistance moment arm)

Even in first gear a hill start is hard and we need a far bit of accelerator. This is why (good) 4WDs have low range gears; badass torque to overcome hills and weight.

Et > Rt = movement (always)
– obviously if we add resistance torque we need to add effort torque for it to remain greater
– Rt = (weightxgravity) x point of resistance on gear


Those fancy lever diagrams visually show the difference between the resistance and effort torques of a particular motion. They have the weight/pull force of R and E and where each pulls compared to the axis, which gives you all the info you need to work out which torque is stronger. If a value is missing the others can be used to find it out.

Eforce x Emoment = Rforce x Rmoment


Applied to the Body
We need to know how to manipulate effort and resistance torque to know how to manipulate the load on patients’ muscles for desired progression; reducing or increasing patient’s muscle work. Like in a car we can manipulate:

Resistance torque by changing
– weight of resistance (literally add or remove weight)
– moment arm of resistance (where we hold/attach weight)

Effort torque by changing
– acceleration (muscle work)
– we can’t change effort moment arm (gears) because our tendons are fixed (although geared mm. would be rad)


HUBS1105 Link
Only other random thing is that we have smaller gears in cars because once we have momentum the resistance torque is much less so we don’t need so much effort torque. We instead need efficient turning about the axis (speed), which would be hindered by a bulky gear.

HUBS1105 talks about limb tethering and this is related to that concept. Since we can’t change gears in the body, having quads at the ankles would hinder the movement with its weight more than it would help move us with its force.

Writing it helped me learn it, hope it helps you too!

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